Adjustment systems or adjustment devices are used, for example, in motor vehicles to adjust windows or sunroofs as driven components. Other applications are, for example, those for the activation of electric tailgates or sliding doors, and therefore for the sake of simplicity reference is made below to driven components, if appropriate without restricting the generality. In such systems, there is furthermore provision, in particular on the basis of legal requirements, for trapping of an object, for example of a person's hand or head, during a closing movement to be detected. This trapping detection is based on a calculated closing course which is calculated, for example, from the rotational speed and the voltage and/or from the current of the electric motor which is provided for the drive. This closing force changes when an object is trapped. This change in the closing force, i.e. a relatively sudden increase in the force beyond a predefined threshold value, serves to make a decision about a trapping situation.
A change in the closing force can, however, also occur if no trapping situation arises, specifically owing to changed conditions during the adjustment movement such as, for example, owing to temperature change and therefore associated changes in frictional forces, in particular in seals, but also owing to contamination in the adjustment system and the like.
It is therefore necessary to be able to differentiate such fluctuations in force owing to changed peripheral conditions from those increases in force which are caused by trapped objects.
In order to be able to make such a differentiation, attempts have already been made to use the difference between the closing force at a specific time and the closing force at a defined preceding time as a criterion instead of the absolute value of the calculated closing force. The distance between the times is selected here such that the system is matched as well as possible to the rigidity of the objects to be detected. The described formation of differences eliminates the static influences; even comparatively small changes in force are therefore “filtered out”. However, it is disadvantageous here that, on the one hand, there is a large requirement for buffering space for the comparison value from the past and that, on the other hand, only a limited section from the past is used for the trapping detection, and therefore information from further back in time is not used (cannot be used).
It would be desirable to be able to carry out the trapping detection on the basis of current force values. However, in this context it is necessary to take account, as mentioned above, of the problem that different (quasi-)static force levels may occur as a result of different mechanical behaviors, that is to say different peripheral conditions; in particular as a result of a corresponding mechanical behavior, the force profiles may exhibit a rising tendency even when trapping does not occur. Such changes in force can occur, for example, when starting, furthermore, in the case of sunroofs, owing to mechanically entrained components, for example wind deflectors and a sun shading means but also owing to system-related parameters such as, for example, heating of the electric motor or, as already mentioned, owing to dirt in the adjustment system.
Usually, a reference value is predefined as a basis for comparison in order then to be able to detect a trapping situation by comparison of the current force value with the reference value or with a threshold value which is permanently related thereto. The reference value is defined when the motor starts and is a constant value in previous anti-trapping systems. If appropriate, predictable reference changes, which are stored as characteristic curves, are taken into account, cf. for example DE 10 2007 050173 B3 or DE 19633941 A1. As has been stated above, owing to changed conditions it is then possible for the real force profile to move away from the anticipated force profile as a result of changes in conditions, in which case it is even possible for trapping to be detected even though no object is trapped. Increased trapping forces would also be possible, which, under certain circumstances, could cause legal requirements to be infringed.